1. Field of the Invention
The present invention relates to a laminated piezo-electric device. More specifically, the invention relates to a laminated piezo-electric device that can be particularly desirably used for the fuel injection devices for automobiles, for the positioning devices used for precision equipment such as optical devices, and as a drive device for preventing vibration.
2. Description of the Related Art
Laminated piezo-electric devices are provided with a pole-like laminate formed by alternately laminating piezo-electric layers and electrode layers (inner electrodes), and can be classified into two types, i.e., those of the co-fired type and those of the stacked type. From the standpoint of lowering the voltage, lowering the cost of production and decreasing the thickness of the layers, the laminated piezo-electric device of the co-fired type is advantageous and is finding gradually spreading applications.
FIG. 10 is a diagram schematically illustrating the structure of a conventional laminated piezo-electric device having a pole-like laminate 53 formed by alternately laminating piezo-electric layers 51 and internal electrode layers 52 in the direction of height. Inert ceramic layers 55 and 55 are laminated on the upper end and on the lower end of the pole-like laminate 53. The inner electrode layer 52 is covered at its one end (side surface) with an insulator 61. As is obvious from FIG. 10, the two inner electrode layers 52 and 52 neighboring each other with the piezo-electric layer 51 sandwiched therebetween have their different ends (side surfaces) covered with the insulators 61. For example, the inner electrode layer 52 laminated on the upper surface of the piezo-electric layer 51 has its right side surface covered with the insulator 61, whereas the inner electrode layer 52 laminated on the lower surface of the piezo-electric layer 51 has its left side surface covered with the insulator 61.
Thus, the insulators 61 are provided on the different side surfaces of the pole-like laminate 53 so as to cover the side surfaces of the inner electrode layers 52 in an alternate manner. As shown in FIG. 10, further, belt-like outer electrodes 70 and 70 are formed on the side surfaces, and lead wires 76 are attached by a solder 77 to the belt-like outer electrodes 70 (one of the belt-like outer electrodes 70, 70 is a positive electrode, and the other one is a negative electrode).
Therefore, the inner electrode layers 52 are connected to the belt-like outer electrode 70 at their side surfaces that have not been covered with the insulators 61. Each of the belt-like outer electrodes 70 and 70 is conductive to every other inner electrode layer 52. Among the inner electrode layers 52, 52 which are neighbored with each other, one of the inner electrode layers 52, 52 is conductive to the belt-like outer electrode 70 of the positive polarity and another of the inner electrode layers 52, 52 is conductive to the belt-like outer electrode 70 of the negative polarity.
Concerning the laminated piezo-electric device having the above-mentioned structure, Japanese Unexamined Patent Publication (Kokai) No. 237172/1992 discloses the one wherein every other end (side surface) of the inner electrode layer exposed to the side surface of the pole-like laminate is covered with a glass (insulator) layer, and the inner electrode layer 52 and the neighboring upper and lower piezo-electric layers (51) are firmly joined together with the glass layer, the glass layer being held in a recessed portion of the outer electrode 70.
In recent years, however, it is a trend to apply a higher electric field and to continuously drive the device for extended periods of time in order to obtain a large amount of displacement under a large pressure by using a small piezoelectric device.
When the above-mentioned conventional laminated piezo-electric device is continuously driven for extended periods of time under the conditions of a high electric field and a large pressure, however, the inner electrode layers 52 formed among the piezo-electric layers 51 peel off the outer electrodes 70 of the positive polarity and of the negative polarity, whereby no voltage is supplied to some of the piezo-electric layers 51 causing a change in the displacement characteristics during the operation. Even the piezo-electric device disclosed in the above-mentioned prior art permits the glass layer (insulating layer) to be cracked when it is continuously driven for extended periods of time under the conditions of a high electric field and a large pressure. Due to the cracking, a short-circuit occurs between the inner electrode layers and the outer electrodes, whereby the voltage is not applied to some of the piezo-electric layers still causing a change in the displacement characteristics during the operation.
That is, the pole-like laminate which is the main body of the laminated piezo-electric device undergoes the stretching and contraction in a direction in which the piezo-electric layers and the inner electrode layers are laminated (in the direction of height). Therefore, the glass layer of a high Young""s modulus covering the ends of the inner electrode layers (also covering the ends of the piezo-electric layers neighboring the inner electrode layers) becomes no longer capable of withstanding the stretching/contraction operation due to the continuous drive for extended periods of time and is destroyed, and a short-circuit occurs between the inner electrode layers and the outer electrodes via the destroyed portions.
Japanese Unexamined Patent Publications (Kokai) Nos. 283451/1995 and 51240/1996 disclose laminated piezo-electric devices having electrically conducting protuberances formed by plating on the ends (side surfaces) of the inner electrode layers 52. With the above piezo-electric devices, however, the junction strength is weak between the electrically conducting protuberances and the pole-like laminate, and the ends of the inner electrode layers peel off the electrically conducting protuberances while the piezo-electric device is in operation. Accordingly, no voltage is supplied to some of the piezo-electric layers like in the above-mentioned case still unable to avoid the problem in that the displacement characteristics undergo a change while the piezo-electric device is in operation.
It is therefore an object of the present invention to provide a laminated piezo-electric device which maintains stable displacement characteristics and excellent durability without permitting the wires to be broken between the outer electrodes and the inner electrodes even after continuously driven for extended periods of time under the conditions of a high electric field and a large pressure.
Another object of the present invention is to provide an injection device by using the above laminated piezo-electric device.
According to the present invention, there is provided a laminated piezo-electric device comprising a pole-like laminate formed by alternately laminating the piezo-electric layers and the inner electrode layers in the direction of height, and a pair of outer electrode plates formed on the different side surfaces of the pole-like laminate, the two inner electrode layers neighboring each other with the piezo-electric layer sandwiched therebetween being electrically connected at their side surfaces to the outer electrode plates which are different from each other, wherein flexible protruded electrically conducting terminals are provided on the side surfaces of the pole-like laminate on where the outer electrodes are arranged, the flexible protruded electrically conducting terminals extending along the side surfaces of the inner electrode layers and being capable of following the stretching and contraction of the pole-like laminate in the direction of height thereof, and the inner electrode layers are joined to the outer electrode plates via the protruded electrically conducting terminals.
According to the present invention, there is further provided an injection device comprising a container having an injection aperture, a laminated piezo-electric device of the invention contained in the container, and a valve for injecting a liquid through the injection aperture being driven by the laminated piezo-electric device.
In the laminated piezo-electric device of the present invention, protruded electrically conducting terminals are provided on the side surfaces on one side of the inner electrode layers (i.e., at portions on the side surfaces of the pole-like laminate), and the inner electrode layers are connected to the outer electrode plate of the positive polarity or to the outer electrode plate of the negative polarity through the protruded electrically conducting terminals. That is, the protruded electrically conducting terminals used in the present invention are flexible to follow the stretching and contraction of the pole-like laminate in the direction of height (in the direction in which the piezo-electric layers and the inner electrode layers are laminated). Therefore, when the piezo-electric device is driven causing the pole-like laminate to undergo the stretching and contraction in the direction of height, stress due to the stretching and contraction is absorbed by the protruded electrically conducting terminals that undergo the deformation. This effectively suppresses the breakage of the electric wires between the outer electrode plates and the inner electrode layers even when the piezo-electric device is continuously operated for extended periods of time under the conditions of a high electric field and a large pressure, avoids a change in the displacement characteristics, and makes it possible to greatly enhance the durability.
Therefore, the injection device which employs the laminated piezo-electric device of the above-mentioned structure operates to stably inject the fuel for extended periods of time.
In the present invention, further, the protruded electrically conducting terminals are formed by applying an electrically conducting paste containing an electrically conducting metal powder and a glass powder onto the side surfaces of the inner electrode layers that are to be connected to the outer electrode plates and onto the side surfaces of the piezo-electric layers positioned in the vicinities thereof, and heating and firing the electrically conducting paste. That is, the glass powder is softened at the time of firing and, in this state, the metal powder that little diffuses into the piezo-electric layers is collected on the side surfaces of the inner electrode layers thereby to form protruded electrically conducting terminals that extend along the side surfaces of the inner electrode layers. Besides, the thus formed protruded electrically conducting terminals are buried at their root portions in the glass layer and are firmly secured. Accordingly, the protruded electrically conducting terminals and the inner electrode layers are firmly joined together, and the protruded electrically conducting terminals are effectively prevented from being peeled off by the movement of the piezo-electric device (by the stretching and contraction of the pole-like laminate).
In the present invention, further, an electrically conducting member for preventing local heating can be provided on the outer surfaces of the outer electrode plates electrically connected to the inner electrode layers via the protruded electrically conducting terminals. The electrically conducting member may be an electrically conducting coil, an electrically conducting corrugated plate, an aggregate of electrically conducting fibers or an electrically conducting sheet formed of an electrically conducting adhesive resin composition. Upon operating the piezo-electric device by supplying a current to the outer electrode plates through such electrically conducting members, it is allowed to prevent the outer electrode plates from being locally heated and to prevent the breakage of wires caused by the local heating even when the piezo-electric device is driven at high speeds by supplying a large current.